The present invention generally relates to implantable medical devices, and to telemetry schemes for percutaneously transmitting end of battery service data from the implantable medical device and even more particularly to telemetry relating to manganese dioxide electrochemical cells.
In implantable medical devices, the desirability to transfer more data at higher speeds has resulted in the percutaneous transmission of data using a radio frequency carrier. The data to be transmitted are of two basic types, namely, analog and digital. The analog information can include, for example, battery voltage, intracardiac electrocardiogram, sensor signals, output amplitude, output energy, output current, and lead impedance. The digital information can include, for example, statistics on performance, markers, current values of programmable parameters, implant data, and patient and unit identifiers.
The earliest RF telemetry systems transmitted analog and digital information in separate formats, resulting in inefficient utilization of the available power/bandwidth. Also, these modulation schemes tended to be less than satisfactory in terms of battery consumption, and did not lend themselves to simultaneously transmission of differing data types.
Many types of RF telemetry systems are known to be used in connection with implantable medical devices, such as cardiac pacemakers. An example of a pulse interval modulation telemetry system used for transmitting analog and digital data, individually and serially, from an implanted pacemaker to a remote programmer is disclosed in U.S. Pat. No. 4,556,063 issued to Thompson et al., herein incorporated by reference. An example of a modern pacemaker programmer for use with programmable cardiac pacemakers having RF telemetric capabilities is disclosed in U.S. Pat. No. 4,550,370 issued to Baker, herein incorporated by reference. An example of voltage telemetry is disclosed in U.S. Pat. No. 4,231,027 issued to Mann et al., herein incorporated by reference. However, the telemetry format which is used under these systems, as well as other prior telemetry systems, have not been entirely adequate.
In particular, it has been noted that certain electrochemical cells utilized in implantable medical devices have changes in voltage that are indicators of impending end-of-service for the cell. However, even when such voltage changes are highly predictable for a particular cells, the telemetry system has been inadequate to give an accurate voltage to reliably predict end-of-service for the cell.
In implantable medical devices which have high current components such as pacemakers with oxygen sensors there is a need for electrochemical cells with a relatively high rate of discharge. This rate of discharge can be provided by cells using an active metal anode and manganese dioxide as the cathode material. However, when manganese dioxide has been used as a cathode material in an active metal anode electrochemical cell it has typically been an anode-limited cell. That is, that a stoichiometric excess of cathode material has been present in the cell so that at the end-of-service for the cell, the anode material is substantially consumed. This produces a cell with a very flat discharge voltage curve. In U.S. Pat. No. 4,399,202 to Ikeda et al., it has been recognized that the slope of the discharge voltage curve at end-of-service can be increased by increasing the relative amount of manganese dioxide in the cell.
However, such a flat discharge voltage curve with a steep slope at end-of-service is not desirable for use in critical implantable medical devices since it could arrive unexpectedly at its end-of-service with adverse consequences for the patient. In fact, it would be desirable in such a cell to provide a highly accurate determination of end-of-service well in advance of arrival of its end-of-service.